Process of preparing polyamide photographic emulsions



Patented May 13, 1 947 PROCESS OF PREPARING POLYAMIDE PHOTOGRAPHICEMULSIONS William W. Watkins, Bufialo, N. Y., assignor to E. I. du Pontde Nemours & Company, Wilmington, Del., a corporation of Delaware NoDrawing. Application April 21, 1944, Serial No. 532,215

1 Claims.

This invention relates to the prepartion of photographic emulsions andemulsion layers. More particularly it relates to the preparation ofphotographic emulsions and emulsion layers which are composed ofwater-sensitive synthetic linear polyamides containing light-sensitivesilver salts.

Gelatin has been used as a binding agent for light-sensitive silverhalides of photographic emulsion layers for many years. It has somedisadvantages, however, one of the major ones being its solubility inwarm water. Thisrequires that the photographic processing of a film orpaper containing a gelatin emulsion be carried out in cool soolutions.Gelatin also deteriorates on storage under humid conditions and for thisreason it is not satisfactory for use in tropical climates. A furtherdisadvantage of gelatin is that it scratches easily both in the wet anddry state. Because of its highly swollen condition when wet, gelatin isvery slow to dry. For rapid drying, heated air is utilized to save time.The drying of gelatin films must be carried out under regulatedconditions because the coating may reticulate if the dew point of theair is not maintained sufhciently high.

A characteristic of gelatin as a medium for photographic emulsions isthat it permits agglomeration of silver particles during developing,washing, fixing and drying of a photographic element giving rise tograininess in the image. This is due to the fact that gelatin swellssome ten times its original thickness during processing in aqueoustreating solutions. This swelling permits some migration of the silverhalide and silver particles.

The object of this invention is to provide a photographic emulsion whichis free from the disadvantages of gelatin. A further object is toprovide a synthetic water permeable photographic layer which does notswell appreciably during photographic processing. A still further objectis to provide a light-sensitive silver halide layer wherein the silversalts do not migrate in a water permeable synthetic colloid bindingmedium. Another object is to provide light-sensitive salt emulsionlayers which can be directly processed to fine-grain images withoutspecial treatment.

It has been discovered that satisfactory photographic emulsions can bemade from water-sensitive synthetic linear polyamides containing manycarbon and nitrogen atoms in the chain of atoms of said polymer byprecipitating light-sensitive silver halides in a viscous solution ofthe polyamides. The resulting emulsions are then washed and cast on asuitable support to form a photographic element.

In a more limited sense the invention is concerned with fine-grainsilver halide emulsions and emulsion layers wherein light-sensitivesilver halides are uniformly and intimately dispersed in awater-sensitive synthetic linear polyamide containing only carbon andnitrogen atoms in the chain of atoms in said polymer which is insolublein water at 20 C. but soluble to the extent of at least 10% by weight inan ethanol-water solution composed of ethanol and 20% water at 60 C. butinsoluble in ethanol-water solutions containing 20% ethanol and 80%water.

The novel emulsion can be advantageously made by dissolving a Watersoluble halide, e. g., lithium bromide in a methanol-water (containing20% water) solution of a suitable polyamide of the above type, e. g.,the interpolymer made by lnterpolymerizing hexamethylene diammoniumadipate, hexamethylene diammonium sebacate and caprolactam; dissolving awater soluble silver salt, e. g., silver nitrate in a similar solventmixture and mixing the two solutions in proper amounts to precipitateall silver as silver halide, e. g., silver bromide. Since the resultingsolution will contain water soluble salts, e. g., lithium nitrate andlithium bromide, it is important to remove these compounds beforeforming a photographic emulsion layer. This may be accomplished bycasting the solution into thin films, washing the same with distilledwater or by setting and shredding or by precipitating in water oracetone, and washing. The washed emulsion is then redissolved, e. g., inmethanol, to form a solution of proper viscosity for casting onto asupport such as a transparent cellulose derivative or polymer film baseor paper, glass, metal, etc.

Before casting to form a photographic element, the emulsion is digestedand can then be modified by the addition of emulsion sensitizers,sensitizing dyes, color formers, etc. Since methanol, ethanol andpropanol, which are suitable solvents for the water-sensitivepolyamides, have a reducing action on silver salts,'the finalphotographic emulsion layers may contain some reduced and. somedevelopable silver salts. These unexposed but developable salts can beresensitized to light by treating the layers with an aqueous solution ofa weak silver-oxidizing agent, e. g., FeCla, HgClg, HgBrz or bromine,etc. Immersion for 10 to 20 minutes in a 5% aqueous bath of suchcompounds is generally suihcient. After drying, the resultingphotographic elements are ready for use. They may be exposed in a cameraor in a printing apparatus, developed, and fixed in the same manners aselements containing gelatinosilver halide layers, whereby fine-grainimages are obtained.

The invention will be further described, but is not intended to belimited by the following examples:

Example I To ten parts of a composition consisting of 20% of aninterpolymer of hexa'nethylene diammom'umadipate,hexamethylenediammoniumsebacate and caprolactam in the weight ratio of40:30:30 in an 80% methanol 20% water solution were added 1.6 parts ofAg'NOs. In an additional 10 parts of the same solution were dissolved1.6 parts of LiBr. Six parts of the latter solution were stirred intothe former solution and the resulting mixture was cast into temporaryfilms of approximately 1 mil thickness. The soluble salts were washedfrom these films with dis tilled water. The fihns were then dried andredissolved in methanol to give a, solution containing 23% polymer basedon polymers plus methanol. This solution was then coated onto atransparent cellulose acetate film base to form a layer of about onemil. in thickness. The element, after drying, was immersed for 15minutes in a 5% aqueous solution of FeCl:;, then washed thoroughly anddried. The resulting film element was exposed in a camera to an objectand then developed in a solution of the following compositioniN-methyl-para aminophenol sulfate grams 2.5

Hydroquinone do 9.0 Sodium sulfite, anhydrous do 75.0 Sodium carbonate,anhydrous do 50.0

' Potassium bromide do 3.5 Water to.. liter 1.0

The films, after fixing in a 25% aqueous solution of crystalline sodiumthiosulfate, were washed and dried. The resulting film element wascharacterized by good contrast, very fine grain and good toughness evenwhen wet. It dried very rapidly and was resistant to abrasion. Theaddition of methanol or ethanol, e. g., 40 to 50%, to the fixing bathaccelerates the fixation.

Example II materially. The resulting solution was cast as temporaryfilms of approximately 1 mil thickness, which were washed, withdistilled water to remove the soluble salts. The Washed films were driedand redissolved in methanol to give a solution containing 23% polymerand the resulting soluti on was coated onto a cellulose acetate filmbase. The resulting film element Was immersed for 15 minutes in a 5%aqueous solution of FeCls, and then washed thoroughly and dried. Th filmelementswerethen exposed and processed in the manner described inExample I. The films so prepared were characterized by having goodspeed, density, and images of good contrast. The particle size in thedeveloped film was much smaller than that obtained in a gelatin film ofthe same speed.

Example III A polyamide composition containing silver nitrate was madeup in the manner described in Examples I and II. A second similarpolyamide composition containing 1.6 parts of. I-lBr in ten partsofsolution was made and five parts of. the latter was. stirred intotheformer solution. To

At the end of this time, the silver halide particle size was found tohave increased the resulting mixture was added 0.1 part of FeBrs. Thesolution was coated onto a celluloseacetate-propionate film base, thenthoroughly washed and dried. The film elements when exposed anddeveloped in the manner described in the preceding examples, evidencedsimilar characteristics to those described in Example I.

This invention is not restricted to the specific polyamides described inExamples I to III, but can be operated with other polyamides havingsimilar characteristics. Suitable additionable polyamides are describedin U. S. Patent 2,285,009.

The percentage of silver salts in the finished film is likewise notlimited to the amounts given in the examples, but may be varied over asubstantial range. The amount to be used depends upon the photographicimage characteristics desired. Amounts from 37%50% of silver halide byweight in the finished film represent a practical range. Various typesof silver halides can be used by varying the salts. Thus, mixtures ofsoluble bromides and/or chlorides can be used by adding mixtures of suchsalts. Other useful soluble halides. include ammonium bromide, potassium bromide, sodium bromide, and their corresponding chlorides andiodides. Other useful soluble silver salts include silver sulfamate,silver sulfate, silver phosphate, and silver acetate.

The emulsions, after formation, are preferably digested at a temperaturefrom 40 to C. for a, period of from 30 minutes to 6 hours. During thedigestion stage or prior thereto, various types of sensitizing agents,e. g., sulfur sensitizers such as allylthioureas, thiocyanates,sodiumthiosulfate allylthiocyanates, can be added. In addition, varioustypes of optical sensitizing dyes which modify the spectralcharacteristics of the resulting emulsions can be added. Suitablesensitizing dyes are described in U. S. Patents 2,010,388, 2,079,376,2,202,990, 2,202,991, 2,202,992, 2,278,461, 2,265,908, etc.

Various types of color-formers or dye intermediates capable of formingquinoneimine or azomethine dyes on color-forming development can beincorporated in the emulsions. Thus, the phenols, naphthols,pyrazolones, acylacetamides, hydrindine, N--homophthalylamines, etC.,particularly those of high molecular weight and are immobile in gelatinemulsion layers can be used. Suitable color-formers are described inUnited States Patents 2,108,602; 2,166,181; 2,178,612;

2,179,223; 2,179,238; 2,179,239; 2,182,815; 2,184,303; 2,186,849;2,200,924; 2,283,276; and 2,328,652.

The photographic elements produced by this invention possess thefollowing advantages (1) Relative insensitivity to humid. and tropicalconditions,

Can be processed at temperatures, in excess of -F.,

Increased resistance toabrasion wet or dry,

Increased resistance to reticulation during photographic processing,

Fine-grain images with ordinary developers,

Freedom from embrittlement;

solution of 1 to 3 carbon atoms containing a water-sensitive polyamidehaving only carbon and nitrogen atoms in the polymer chain, admixing anaqueous solution of a water soluble halide digesting the resultingemulsion at a temperature of 80 C. for several hours, casting theemulsion into films, washing said films, redissolving aforesaid films inan aqueous-alkanol solution and coating said solution onto a support,said polyamide being insoluble in water at 20 C. but soluble to theextent of at least 10% by weight at 60 C. in an ethanol-water solutioncomposed of 80% ethanol and 20% water but insoluble in such a solutioncomposed of 80% water and 20% ethanol. I

WILLIAM VT. WATKINS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 323,576 Kuhne May 4, 19432,289,775 Graves July 14, 1942 2,299,839 McQueen Oct. 27, 1942 229,694Graf July 6, 1880 2,365,416 Kuhne Dec. 19, 1944 FOREIGN PATENTS NumberCountry Date 218,382 Switzerland Mar. 16, 1942 496,049 Great BritainNov. 21, 1938

